1. Field of the Invention
The present invention relates to a process for producing homogeneous acrylonitrile/methacrylonitrile copolymers that are melt processable. More specifically, the invention relates to a monomer starved process for producing acrylonitrile/methacrylonitrile copolymers in which the polymerization rate exceeds or equals the addition rate of the comonomers of acrylonitrile and methacrylonitrile.
2. Description of the Prior Art
Acrylic polymers are desirable to produce fibrous textiles, films, molded objects, packaging applications, and the like because the acrylic polymers contain mostly nitrile polymers. Nitrile polymers have excellent barrier properties, chemical resistance, rigidity, heat resistance, UV resistance, moisture retention, and bacteria resistance. Acrylonitrile and methacrylonitrile monomers are nitrile monomers useful to produce acrylic polymers.
In the past, acrylic polymers containing greater than about 20% by weight polymerized acrylonitrile were not melt processable. U.S. Pat. No. 3,565,876 discloses that up to about 20% by weight of an acrylonitrile monomer can be copolymerized with a methacrylonitrile monomer to form an extrudable copolymer which can be readily oriented and possesses excellent physical properties. Increasing the acrylonitrile content above about 20% by weight in the acrylonitrile/methacrylonitrile copolymer results in a resin which is thermally unstable and not processable by any standard economical commercial melt processing techniques including extrusion. Accordingly, other non-nitrile monomers have been added to the acrylic polymers to make the acrylic polymers melt processable. High acrylic polymers are commonly processed by solvent techniques such as wet spinning acrylic fibers. The use of solvents is disadvantageous because the solvents must be removed from the acrylic polymer prior to end use resulting in voids in the acrylic polymer. Furthermore, the solvents are typically toxic and their disposal has negative environmental implications.
U.S. Pat. No. 5,106,925 discloses a process for producing an acrylonitrile/methacrylonitrile copolymer that is melt processable in the absence of solvent. In the patent the acrylonitrile/methacrylonitrile copolymer is produced under flooded monomer process conditions. In the flooded monomer process, the molar weight ratios of the comonomers of acrylonitrile and methacrylonitrile must be carefully controlled and adjusted throughout the polymerization process in relationship to the conversion of polymer. The polymer conversion is tracked throughout the process so that the addition of the comonomers may be adjusted to obtain the desired polymer. Thus, this process is disadvantageous because of the necessity to track, to predict the polymer conversion rate, and to adjust the rate of addition of the comonomers throughout the process.
It is advantageous to produce acrylonitrile/methacrylonitrile copolymers by a process in which the comonomer feed ratio is fixed throughout the polymerization process. Further, it is desirable to produce homogeneous acrylonitrile/methacrylonitrile copolymers with excellent barrier properties and thermoplastic properties which copolymers are melt processable in the absence of a solvent.